The present invention is directed to coating technology and, more particularly, to pulsed detonation coating.
Several techniques have been used to implement thermal spray coating. One approach has been High Velocity Oxygen/Fuel System (HVOF), in which solid particles are injected in high velocity gas produced by reaction of oxygen and a fuel at high pressure. Such systems typically are used for deposition at atmospheric pressure and primarily are used for coating metal alloys and WC/Co powders with particle sizes larger than about 10xcexc. Other thermal spray coating techniques include plasma spray, in which particles are heated and accelerated by high temperature plasma produced by an electric discharge in an inert gas atmosphere. Plasma spray systems have been used for both atmospheric- and low-pressure coatings.
Plasma coating and HVOF coating suffer from several disadvantages. One disadvantage is the inability of directly using nanosized powder, or even small micron size ( less than 10 xcexcm) particles. This is due to the small particles closely following the streamlines of the carrying gas and decelerating significantly in the stagnation region of the jet/substrate interaction. The decelerated particles are susceptible to being diverted by the flow in the stagnation region, and are not deposited on the substrate. To help alleviate this problem, nanosized powders typically are post-processed to create 10-60 xcexcm agglomerates that retain nanostructure and are strong enough to survive the jet environment before deposition. However, agglomeration not only adds to processing cost, but also can promote grain growth, contamination, and other deterioration of the original powders.
Thermal spray coating also has been implemented by intermittent detonations, e.g., by the use of a detonation gun (D-Gun). D-guns can be used for coating a wide variety of materials, such as metals, cermets, and ceramics. D-guns typically have a relatively long (often about 1 m), fluid-cooled barrel having a small inner diameter of about one inch. Typically, a mixture of reactive gases, such as oxygen and acetylene, is fed into the gun along with a comminuted coating material in two phases. The reactive gas mixture is ignited to produce a detonation wave, which travels down the barrel of the gun. The detonation wave heats and accelerates the coating material particles, which are propelled out of the gun onto a substrate to be coated.
The detonation wave typically propagates with a speed of about 2.5 km/sec in the tube and can accelerate the particle-laden detonation products to a velocity of about 2 km/sec. However, coating particles never reach the velocity of detonation products due to inertia. In practice, particle velocities usually are lower than about 900 m/sec. The temperature of the detonation products often reaches about 4000 K. After the coating material exits the barrel of the D-gun, a pulse of nitrogen typically is used to purge the barrel. Newer designs of the D-guns allow operation frequencies of up to about 100 Hz. See, e.g., I. Fagoaga et al., xe2x80x9cHigh Frequency Pulsed Detonation (HFPD): Processing Parametersxe2x80x9d (1997).
One example of a gas detonation coating apparatus is illustrated in U.S. Pat. No. 4,669,658 to Nevgod et al. A barrel enclosed in a casing has annular grooves made on an inner surface of an initial portion thereof. A main pipe housing a spark plug and having annular grooves on its inner surface is inserted into the initial portion of the barrel. In operation, a gas supply means is turned on. The apparatus works in cycles, each cycle accompanied by gas flowing into the barrel and the main pipe through tubes, gas conduits, and additional pipes. After the gases fill the barrel, the gas mixture is ignited in each cycle with the aid of the spark plug. The detonation products are said to quickly heat up the walls of the barrel and the annular grooves.
According to Nevgod, the gases flowing into the barrel are heated up in two stages. During the first stage the gases are warmed up in the additional pipes heated up in cycles by the detonation products. The heat insulation tubes are said to prevent the pipes from cooling down. During the second stage, the gases are heated up in the barrel and partially in the main pipe. The annular grooves on the inner cylindrical surface of the initial portion of the barrel, the inner surface of the main pipe and on the inner surface of the cover on the end of the barrel, are said to enhance the efficiency of heat exchange with the gases due to an increase in the heat exchange area and due to gas turbulization. The gases are heated to a temperature approximating that of self-ignition. A plurality of ignition sites is provided to accelerate the burning process.
Presently available detonation coating technology suffers from several drawbacks. One major drawback is that the D-guns are bulky, with the barrel alone often being as much as 1 m in length. Because of the difficulties associated with handling the bulky D-gun, the D-gun often is held stationary while the substrate to be coated is moved relative to the barrel of the D-gun. This is especially problematic for coating larger-sized articles that cannot easily be moved. Another drawback is that the coating rate is limited by the relatively low operation frequencies. Increase of operation frequency is possible if a shorter barrel is used. However, a shorter barrel leads to decrease in particle velocity, due to shorter cycle time available for particle acceleration by detonation products, which in turn will reduce coating quality. Reduction of particle size to less than 10xcexc will help particle acceleration by high-speed detonation products, however these particles will quickly decelerate in the stagnation layer when approaching the coated substrate, and thus will arrive to the substrate at low velocity.
It would be desirable to develop thermal spray coating technology that enables the use of a smaller coating apparatus, especially one that can be adapted for coating the insides of tubes and other difficult-to-reach areas. Because for many applications coating quality improves with reduction of grain and particle size and with increase of particle impact velocity, it would be desirable to directly coat small-micron, sub-micron, and nanoscale particle that impact the substrate at very high velocities. It would be desirable to directly coat small-micron and sub-micron sized particles at very high velocities to give high quality cold coatings or impact coatings. It would be desirable to minimize the amount of local heating of the substrate surface during coating to enable the coating of very thin and/or low melting point substrates.
The present invention is directed to a method and apparatus for producing a coating on a substrate situated in vacuum or low pressure by pulsed detonation coatings. A pulsed detonation gun comprises a detonation chamber having ignition means and an outlet nozzle. In one preferred embodiment, a detonable mixture containing the coating precursor is formed in the detonation chamber. The detonable mixture is ignited to produce detonation products containing the coating precursor. Following detonation, the detonation products containing the coating precursor particles are discharged through the nozzle and expand at high velocities in a vacuum or low-pressure chamber. The coating precursor particles are heated and accelerated toward a substrate to produce a high quality coating.
According to another preferred embodiment, a suspension of a coating precursor in a detonable fuel is injected into a detonation chamber to form a detonable mixture. The detonable mixture is ignited to produce detonation products containing the coating precursor. The detonation products containing the coating precursor are discharged from the detonation chamber and accelerated in a low-pressure chamber. The detonation products containing the coating precursor are contacted with a substrate to produce a coating on the substrate.
According to an alternative embodiment of the invention, a detonation chamber has a first region containing ignition means and a second region having a nozzle at an end portion thereof. A detonable mixture is injected into the first region of the detonation chamber. A coating precursor, which can be provided in an inert gas or in a suspension, is injected into the second region of the detonation chamber. The detonable mixture is ignited, creating a wave front that accelerates the coating precursor through the nozzle into a low-pressure chamber and into contact with a substrate to produce a coating.
The vacuum or low-pressure environment of the present invention provides a greater pressure gradient for the detonation product, e.g., compared to conventional pulsed coating processes performed at atmospheric conditions. The greater pressure gradients yield higher particle acceleration and velocities, which translate into the ability to reduce the overall size of the apparatus and improve coating quality. The apparatus of the present invention thus can be made substantially smaller in size than the conventional, bulky D-guns. The apparatus can be made portable and can be easily manipulated to accurately apply coatings to substrates having a wide variety of sizes and shapes. Maintaining low pressures near the substrate enables coatings of smaller particles, e.g., small micron or nanosized particles, by avoiding interference from the carrying gas, which will be at low pressure and low density at the substrate.
Local heating of the substrate can be reduced or substantially avoided by reducing the duration of the detonation period, i.e., the time of exposure to the high temperature detonation products. Preferably, any appreciable heating of the substrate is restricted to a very thin surface portion thereof. Optionally, the substrate surface can be subjected to rapid temperature quenching after each exposure to further minimize local heating. Reduced local heating advantageously enables the coating of very thin and/or low melting point substrates and permits higher coating rates.